This invention relates to a hydrodynamic vortex separator, and is particularly concerned with such a separator for use in separating floatable matter, such as grease, from a liquid flow which comprises the floatable matter. The invention also relates to a separator for separating from each other, and from the liquid stream in which they are present, a settleable material and a floatable material such as grit and grease present in a waste water stream. Also disclosed herein is a new inlet arrangement for a hydro-dynamic separator.
Hydrodynamic vortex separators are well known and are based on initial research work carried out in the 1950""s and 1960""s (Design, Construction and performance of vortex overflows, Bernard Smisson, Symposium on Storm Sewage Overflows, Institution of Civil Engineers, 1967, pages 99-110). They have found application as combined sewer overflows (CSOs) and as grit separators.
Separators known as xe2x80x9cHydro-Dynamicxe2x80x9d separators are low energy devices which operate on the principle of allowing a liquid containing suspended solid material to rotate in a cylindrical vessel so that the solid material falls under gravity to the base and there is swept to a central lower outlet by an inward sweeping effect caused by complex flow patterns in the device. It is known that the device is suitable for providing enhanced settlement of solids material from a liquid/solid mixture. Thus, such devices have been used in sewage treatment for separating hard grit from the incoming raw sewage, with the resultant degritted sewage then being passed to a conventional sewage treatment plant. They are also used as xe2x80x9cstorm water overflowsxe2x80x9d upstream of conventional sewage treatment works to ensure that gross contamination is separated from liquid waste during storm conditions when the sewage treatment works is unable to cope with the high flow. xe2x80x9cHydro-Dynamicxe2x80x9d separators of this type are described and claimed in, for instance, British Patent Specifications Nos. 2082941 (corresponding to U.S. Pat. No. 4,451,366) and 2,158,741 (corresponding to U.S. Pat. No. 4,747,962).
The known hydro-dynamic separator is a simple device with no moving parts. The simple geometry of the device however, hides an internal complexity of flow structure. The mean flow pattern observed is a downward helical flow in the outer region and an upward helical flow near the central region of the separator. These two spiral flow regimes are separated by a shear zone region. The combination of underflow and overflow leads to a non-uniform axial flow profile. The effects of fluid viscosity, boundary layers and momentum transfer between adjacent zones of flow moving at different velocities, cause velocity gradients and vorticity (rotation) to be present. These result in a secondary flow, superimposed on the primary flow, which in turn results in solids being swept towards the underflow channel (or solids collection trough/hopper). The hydraulic regime in the separator ensures very little short-circuiting with a near plug-flow type flow regime.
As discussed above, the principle of hydrodynamic separation has heretofore been used to facilitate the separation of settleable material (i.e. material which tends to settle under the action of gravity) from a liquid flow. It has now surprisingly been found that this same principle may be used for the separation of a floatable material from a liquid flow. Put simply, this is achieved by arranging the elements of the known hydrodynamic separator in the opposite orientation. Surprisingly, the flows generated in use act to concentrate floatable solid matter, which naturally rises in the vessel as a result of its buoyancy, at a central upper region where it can be directed away from the normal flow of liquid through the vessel, and collected for disposal.
Thus, according to a first aspect of the present invention, there is provided a hydrodynamic separator for the treatment of a liquid flow containing floatable material to separate the floatable material from the liquid containing it, said separator comprising a separating vessel having:
a cylindrical outer wall;
an inlet means for introducing said liquid into the vessel in a manner to promote a rotational flow of liquid in the vessel;
a base at one end;
an upper wall at the end opposite the base, the upper wall including an axial outlet opening for receiving a flow containing floatable material separated from the liquid flow to the vessel; and
an outlet means, separate from the outlet opening in the upper wall, comprising a conduit for removing a primary liquid flow from the vessel, which conduit communicates with the interior of the vessel at a substantially axial location between the base and the upper wall.
The separator of the first aspect of this invention may further include an annular dip-plate spaced from the outer wall of the vessel. The dip plate helps in stabilising the flows in the vessel and in particular delimits an outer faster moving flow and an inner slower moving flow. The precise radial position of the dip plate may be chosen to give the optimum performance for a given design of separator, depending on the likely character of the liquid flow to the vessel. Normally, the annular dip plate will be located between about 0.2 and 0.8 of the distance between the axis of the vessel and the outer wall. The height of the dip plate is not critical. In some embodiments it will be relatively shallow in height relative to the height of the vessel; in other embodiments, however, it may be quite deep. Normally, the dip plate will be of a height which is at least the height of the inlet opening into the vessel and normally it will at least partially overlap the inlet opening in the vertical direction.
The separator of this aspect of the invention may also comprise a flow-modifying member provided within the vessel to define with the top wall an annular opening which is spaced from the outer wall. The axial outlet in the top wall referred to above opens inward of the said annular opening. The said flow modifying member body may be generally conical in form with its base uppermost, and aligned about the central axis of the vessel; the cone may be hollow or solid and may be provided with a central opening extending through the cone and aligned with its axis. In preferred embodiments, the cone is frusto-conical, that is to say with a truncation plane parallel to the base.
The upper wall preferably slopes upwardly towards the annular opening at its central region. The annular opening should preferably be positioned between about 0.2 and 0.8 times the radius of the vessel, more preferably between about 0.4 and 0.6 times the radius of the vessel from the central vertical axis.
The axial outlet opening in the upper wall may communicate with a chamber above the vessel, for collecting floatable matter. This chamber may include an outlet means whereby said floatable matter may be removed. Alternatively, the axial outlet opening may communicate directly with a suitable conduit whereby the floatable matter may be continuously directed away from the separator.
The base of the vessel preferably slopes downwards towards the central axis of the vessel. The base may include an axial outlet opening inward of the annular opening for receiving a flow containing settleable matter separated from the liquid flow to the vessel. This outlet opening may communicate with a chamber or sump which is itself provided with a outlet via which settled matter may be conveyed away from the vessel.
With a separating device having the features specified, when a liquid containing floatable matter is introduced in the vessel and caused to circulate about the vertical axis of the vessel, a complex flow pattern is established which gives rise to an efficient separation of floatable matter. The complex flow patterns established can be simplified and expressed as a circulating flow about the central vertical axis of the vessel, the circulating flow being divided between an outer, relatively fast flow and an inner relatively slow flow, the shear zone between these two regions preferably being stabilised by the upper edge of the annular dip-plate and the edge of the said flow-modifying member or body which defines the mouth of the annular opening.
The majority of the flow of liquid into the vessel flows out of the vessel via said outlet means. In addition to the majority of the liquid flow, any neutrally buoyant materials are also collected by and removed via the said outlet conduit. As stated above, the outlet means comprises a conduit which communicates with the interior of the vessel to receive liquid flow independent of the flow towards the axial opening in the top wall of the vessel. This conduit may communicate with a point in the vessel which is substantially located at or around the axis of the vessel so that liquid in the vessel following a toroidal path is captured by an open end of the conduit and carried away, for example substantially vertically, from the vessel to a point located outside the vessel.
The inlet means may comprise an inlet opening in the outer wall which communicates with an inlet conduit outside the vessel.
In one embodiment, the inlet conduit is tangentially oriented with respect to the outer wall in order to direct liquid into the vessel in a manner to promote a circulating flow in the vessel.
In another embodiment, the inlet means may comprise, within said vessel and communicating with said inlet opening, a duct arranged to direct incoming liquid flow in an arcuate path adjacent the inner surface of the outer wall of the vessel thereby to create said rotational flow.
In this embodiment, the said duct may comprise a vertical wall (preferably arcuate) inward of the outer wall of the vessel, extending part way round the vessel (this may be, for example, up to about a quarter of the circumference of the vessel) and a base wall forming a lower barrier to said duct. Thus, together, said arcuate wall, the outer wall of the vessel and the base wall form said duct, which may be open along its top side. The arcuate wall may be parallel or generally parallel to the outer wall of the vessel. Alternatively, said arcuate wall may intersect the outer wall at its inlet opening end and, as a result of having a smaller radius of curvature than the outer wall, follow a path which diverges from the outer wall. Further, the base wall may slope downwardly in a direction away from the inlet opening.
In one presently preferred arrangement of this embodiment, the inlet opening into the vessel is provided in the outer wall of the vessel above the upper wall, communicating with an inlet chamber which lies above the inlet duct and which communicates with the duct via a slot in the said top wall of the vessel. The liquid flow therefore sinks from this chamber through the slot and into the inlet duct and is there directed by the base-of the duct and the associated arcuate wall into the vessel with a-rotational component to its flow.
The inlet chamber may extend towards the outlet, beyond the point at which the inlet duct ends. A baffle may be provided to divide the inlet chamber at or around the point at which the inlet duct ends the baffle terminating above the floor of the inlet chamber so allowing liquid flow between two separate portions of the inlet chamber. The open part between the two portions may be provided with a screen to prevent floating matter from passing from the inlet side of the inlet chamber to the outlet side of the inlet chamber. The screen may comprise a series of spaced apart vertical bars. At the outlet side of the inlet chamber may be provided with an overflow weir which may be adjustable in height. The weir communicates with the outlet. This arrangement allows the overflow of liquid from the inlet to the outlet under high flow conditions beyond those at which the separator operates.
As an alternative to the arrangement described in the immediately preceding paragraph, the inlet chamber may be isolated from the outlet by a baffle having an opening which is normally closed by a closure, which closure is openable under relatively high flow conditions to allow the passage of liquid flow from the inlet chamber to the outlet. For example, the opening may be provided with a flap means which is normally biassed to close the opening. The biassing of the closure is selected, however, such that the flap opens under high flow conditions when a sufficient head of liquid has accumulated in the inlet chamber. As an alternative, the closure may be float operated, the float being position to float on the liquid level in the inlet chamber and being set to open the closure when the liquid level in the inlet chamber reaches a predetermined height. In this embodiment, the opening in the baffle may further be provided with a non-return means to prevent return flow of liquid from the outlet side of the baffle to the inlet chamber.
The preferred arrangement of inlet described above has a particular advantage over inlet arrangements to the known types of hydrodynamic separators, which is that the inlet opening to the vessel may be disposed at a height which is substantially the same height above the base of the vessel as the outlet from the vessel. In prior arrangements, the inlet opening has been located at a point in the middle of the side wall of the vessel, well below the level of the outlet, and this has necessitated a so-called inlet pit in the inlet line to the separator; under full flow conditions, the liquid in the inlet pit adopts a level which corresponds to the level of the outlet. A better appreciation of this advantage may be seen in FIG. 7a/b. 
In this embodiment, the inlet conduit outside the vessel need not be tangential to the vessel, and indeed in a preferred embodiment it is not tangential. For example, the inlet conduit may intersect the outer wall of the vessel at a right angle or substantially at a right angle.
The separator in accordance with the present invention may additionally comprise the features of a conventional hydrodynamic separator, whereby it may perform the roles of separating floatable matter and solid, settleable matter, as well as neutrally buoyant material which are removed with the primary liquid flow out of the vessel. In such an embodiment, the separator of the present invention may additionally comprise a flow-modifying body within the vessel which defines with the base an annular opening which is spaced from the outer wall; the annular opening should preferably be positioned between about 0.2 and 0.8 times, preferably between 0.4 and 0.6 times, the radius of the vessel from the central vertical axis. The body may be in the form of a right, circular cone which is aligned about the central axis of the vessel with its base being lowermost; the cone may be hollow or solid and may be provided with a central opening extending through the cone and aligned with its axis. In preferred embodiments, the cone is frusto-conical, that is to say with a truncation plane parallel to the base. In addition for this embodiment, the annular dip-plate may be adjusted in height and radial position, if necessary, so as to assist in stabilising flow patterns which enhance settlement of settleable matter at the base of the vessel.
The inlet means described above in relation to the first aspect of the invention may also be used as an inlet means for a conventional hydro-dynamic separator as described herein.
Thus, according to a second aspect of the present invention, there is provided a hydrodynamic separator for the treatment of a liquid flow containing settleable matter from the liquid containing it, the separator comprising a vessel having:
a cylindrical outer wall;
an inlet means for introducing said liquid into the vessel in a manner to promote a rotational flow of liquid in the vessel, said inlet means comprising an inlet opening and, within said vessel in communication with said inlet opening, a duct arranged to direct liquid flow in an arcuate path adjacent the inner surface of the outer wall of the vessel thereby to create said rotational flow; and
a base at one end, wherein the base includes an axial outlet opening for receiving a flow containing settleable matter separated from the primary liquid flow.
The separator may also include an annular dip-plate which is spaced from the outer wall of the vessel as described above and a flow-modifying member, analogous to that element as described above in relation to the first aspect of this invention, provided within the vessel to define with the base an annular opening which is spaced from the outer wall and which is outward of the annular opening in the base.
The separator of the first aspect of the present invention may be used in a method of separating floatable matter from a liquid flow containing said floatable matter (and in some embodiments also for separating from the liquid flow settleable matter and/or neutrally buoyant matter). In said method, the liquid flow is introduced into the vessel via the said inlet means and caused to flow in a rotational manner about the axis of the vessel. The flow patterns created cause floatable matter to be directed towards and through the outlet opening in the top wall, from where it can be removed. Settleable matter may be removed via the outlet opening in the base and neutrally buoyant material may be removed with the main liquid flow through the said outlet means.
In such methods of use, the hydraulic head needed to drive the flow through the device is less than 1000 mm and typically less than 500 mm This classifies the method as xe2x80x9clow energyxe2x80x9d by contrast with hydrocyclones which are high energy devices and typically operate at pressure heads of several bar (One bar is 10 m of water head).